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SubjectRe: [PATCH 5/5] proc: export more page flags in /proc/kpageflags

* Wu Fengguang <fengguang.wu@intel.com> wrote:

> Export 9 page flags in /proc/kpageflags, and 8 more for kernel developers.
>
> 1) for kernel hackers (on CONFIG_DEBUG_KERNEL)
> - all available page flags are exported, and
> - exported as is
> 2) for admins and end users
> - only the more `well known' flags are exported:
> 11. KPF_MMAP (pseudo flag) memory mapped page
> 12. KPF_ANON (pseudo flag) memory mapped page (anonymous)
> 13. KPF_SWAPCACHE page is in swap cache
> 14. KPF_SWAPBACKED page is swap/RAM backed
> 15. KPF_COMPOUND_HEAD (*)
> 16. KPF_COMPOUND_TAIL (*)
> 17. KPF_UNEVICTABLE page is in the unevictable LRU list
> 18. KPF_HWPOISON hardware detected corruption
> 19. KPF_NOPAGE (pseudo flag) no page frame at the address
>
> (*) For compound pages, exporting _both_ head/tail info enables
> users to tell where a compound page starts/ends, and its order.
>
> - limit flags to their typical usage scenario, as indicated by KOSAKI:
> - LRU pages: only export relevant flags
> - PG_lru
> - PG_unevictable
> - PG_active
> - PG_referenced
> - page_mapped()
> - PageAnon()
> - PG_swapcache
> - PG_swapbacked
> - PG_reclaim
> - no-IO pages: mask out irrelevant flags
> - PG_dirty
> - PG_uptodate
> - PG_writeback
> - SLAB pages: mask out overloaded flags:
> - PG_error
> - PG_active
> - PG_private
> - PG_reclaim: mask out the overloaded PG_readahead
> - compound flags: only export huge/gigantic pages
>
> Here are the admin/linus views of all page flags on a newly booted nfs-root system:
>
> # ./page-types # for admin
> flags page-count MB symbolic-flags long-symbolic-flags
> 0x000000000000 491174 1918 ____________________________
> 0x000000000020 1 0 _____l______________________ lru
> 0x000000000028 2543 9 ___U_l______________________ uptodate,lru
> 0x00000000002c 5288 20 __RU_l______________________ referenced,uptodate,lru
> 0x000000004060 1 0 _____lA_______b_____________ lru,active,swapbacked

I think i have to NAK this kind of ad-hoc instrumentation of kernel
internals and statistics until we clear up why such instrumentation
measures are being accepted into the MM while other, more dynamic
and more flexible MM instrumentation are being resisted by Andrew.

The above type of condensed information can be built out of dynamic
trace data too - and much more. Being able to track page state
transitions is very valuable when debugging VM problems. One such
'view' of trace data would be a summary histogram like above.

( done after a "echo 3 > /proc/sys/vm/drop_caches" to make sure all
interesting pages have been re-established and their state is
present in the trace. )
The SLAB code already has such a facility, kmemtrace: it's very
useful and successful in visualizing complex SLAB details, both
dynamically and statically.

I think the same general approach should be used for the page
allocator too (and for the page cache and some other struct page
based caches): the life-time of an object should be followed. If we
capture the important details we capture the big picture too. Pekka
already sent an RFC patch to extend kmemtrace in such a fashion. Why
is that more useful method not being pursued?

By extending upon the (existing) /proc/kpageflags hack a usecase is
taken away from the tracing based solution and a needless overlap is
created - and that's not particularly helpful IMHO. We now have all
the facilities upstream that allow us to do intelligent
instrumentation - we should make use of them.

Ingo


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